A polymer having such a function as optical compensation can be obtained by polymerizing a polymerizable compound having a liquid crystal phase. The function is exhibited by fixing the alignment of the liquid crystal molecules through polymerization. Several polymerizable compounds are developed for utilizing such the function of polymerizable compound. However, the function may not be sufficiently exhibited with only one kind of polymerizable compound. Accordingly, such attempts have been made that a composition is prepared with plural kinds of polymerizable compounds, and the composition is polymerized to provide a polymer (see JP-A-2006-307150 and JP-A-2004-231638).
In this specification, the fact that a liquid crystal skeleton has an alignment state, such as a homogeneous alignment, a tilted alignment, a homeotropic alignment, a twisted alignment or the like may be referred to as “having a homogeneous alignment”, “having a tilted alignment”, “having a homeotropic alignment”, “having a twisted alignment” or the like, respectively. For example, a liquid crystal film having a homogeneous molecular alignment, i.e., a homogeneous-aligned liquid crystal film, may be referred to as a liquid crystal film having a homogeneous alignment.
A polymer having a homogeneous alignment can be used in combination, for example, with a ½ wavelength plate, a ¼ wavelength plate or a film having another optical function (see JP-A-2002-372623).
In the aforementioned use, there are case where the polymerizable liquid crystal material is laminated on a glass substrate, a glass substrate which is coated on the surface thereof with a plastic thin film (such as an overcoated film formed on a color filter), a color filter substrate (see JP-A-2006-285014) or a plastic substrate. Examples of a material used as the plastic substrate include such polymers as TAC (triacetyl cellulose), polycarbonate, PET (polyethylene terephthalate) and a cycloolefin resin.
The inventors have found a polymerizable liquid crystal composition that can stably maintain a liquid crystal phase at room temperature and shows uniform alignment property (see JP-A-2007-16213). However, the composition may sometimes be difficult to control the optical characteristics thereof, particularly birefringence (Δn), and thus leave room for improvement. As a method for controlling birefringence, utilization of a compound having an aromatic ring in the short axis direction of mesogen (a compound having a triptycene ring) has been proposed, but it is still difficult to control birefringence by the proposal (see JP-A-2006-111571).